1. Field of the Invention
The present invention relates to an electronic photographic image forming apparatus, and, more particularly, to an orthogonal transfer type image forming apparatus where a toner image carried by a toner image carrier belt is transferred in a direction orthogonal to the transporting direction of a print form.
2. Description of the Prior Art
Recently, image forming systems increasingly use toner as represented by a laser printer which can produce a clear image with a higher resolution.
FIG. 3 is a schematic view showing an arrangement of a conventional image forming apparatus; FIG. 4 is a perspective view showing transfer and transporting processes in the conventional image forming apparatus; and FIG. 5 is a schematic view showing the transfer process through movement of a transfer opposite roller in the conventional image forming apparatus.
As shown in FIGS. 3 and 4, a photosensitive material belt 1 is resiliently deformable, coated with an organic photoconductive material on its surface, and wound in a loop around a support roller 9a, a displacing roller 9b, and the transfer opposite roller 8 to be capable of running in the direction of the arrows in the figure. The transfer opposite roller 8 is arranged to be movable on a plane at the same level with respect to a transfer plate 10, and to rotate by itself.
Arranged in the running direction of the photosensitive material belt 1 are a cleaning unit 4 for removing residual toner after transfer, a discharging unit 5 for discharging residual charges after transfer, a charging unit 2 for uniformly charging the photosensitive material belt 1 to negative potential for preparation to transfer, an exposure unit 6 for forming an electrostatic latent image on the surface of the photosensitive material belt 1 by projecting exposure light 6a such as a laser beam, and a developer unit 3 containing toner 3a therein in this order. In addition a print form 7 is fed onto the transfer plate 10 by a sheet feed roller (not shown) disposed in a path at the insertion end of the transfer plate 10, and is fed out to a fixing unit 15 by a sheet discharge roller (not shown) disposed in a path at the discharge end after transfer.
Here, as the exposure unit 6, a type using an LED or LCD for projecting light has been employed in place of exposure light 6a using a laser. In addition, the fixing unit 15 is a non-contact type comprising a heat source 12, a reflector plate 13 and a transport plate 14, thereby heating and fixing a toner image on the print form 7.
The loop of the photosensitive material belt 1 is, as shown in FIG. 3, in a direction orthogonal to the path in which print form 7 is supplied from the upstream and fed toward the fixing unit 15, so that the running direction of the photosensitive material belt 1 is in a relationship orthogonal to the supply direction of print form 7. Then, at a time of forming an image, the toner image 3c is formed on the surface of the toner image carrier belt 1 in accordance with movement of the photosensitive material belt 1 in the direction of the arrows.
In such an orthogonal transfer system, the transfer of a toner image 3c to the print form 7 is performed by stopping the running of the photosensitive material belt 1 and the transportation of the print form 7, then moving the transfer opposite roller 8 in the direction of the arrow in FIG. 5 along the transfer plate 10 while rotating it with respect to the print form 7 held stationary on the transfer plate 10. This movement of the transfer opposite roller 8 causes sequential deformation of the photosensitive material belt 1 as shown in the figure, so that its surface rubs the print form 7. As the photosensitive material belt 1 is deformed, the displacing roller 9b also moves from its initial position indicated by a broken line in the figure to a position indicated by a solid line. Accordingly, such movement of the displacing roller 9b prevents the photosensitive material belt 1 from being unnecessarily loosened.
In such transfer by the movement of the transfer opposite roller 8 and the deformation of the photosensitive material belt 1, the transfer opposite roller 8 passes over the print form 7 and moves outside the print form 7 every time one transfer process is completed. That is, as shown in FIG. 3, when the transfer opposite roller 8 is positioned outside the print form 7 from its left edge, the photosensitive material belt 1 is in a state where it is not in contact with the print form 7. Under such state, the print form 7 is fed. The amount of feed for the print form 7 at the moment is equal to the effective transfer width of the photosensitive material belt 1. Therefore, transfer is repeated for one sheet of print form 7 by intermittently feeding the print form 7, and sequentially forming toner images 3c on the photosensitive material belt 1.
Incidentally, when the transfer opposite roller 8 shown in FIG. 3 moves from the initial position to the right as shown in FIG. 4 through rotation, the displacing roller 9b disposed in the advancing direction of the transfer opposite roller 8 is freely displaced from its position under a load from deformation of the photosensitive material belt 1.
Therefore, tension by the transfer opposite roller 8 and the displacing roller 9b is maintained substantially constant for the photosensitive material belt 1 positioned between the transfer opposite roller 8 and the displacing roller 9b.
On the other hand, there is no compensation function for tension against the movement of the transfer opposite roller 8 for the photosensitive material belt 1 positioned between the transfer opposite roller 8 and the support roller 9a because the support roller 9a is fixed in position. Thus, there arises insufficient tension on the photosensitive material belt 1 between the support roller 9a and the transfer opposite roller 8, so that downward flexural deformation D would be generated as shown in FIG. 5.
Once such flexural deformation D is generated, the photosensitive material belt 1 may have minute flexure or tend to contact with the print form 7 due to the downward load from the flexural deformation D at a section immediately after the photosensitive material belt 1 passes through a nip between the transfer opposite roller 8 and the print form 7. Therefore, there is a possibility that the photosensitive material belt 1 at the position immediately after the nip contacts with a toner image 3d immediately after printing on the print form 7 to disturb the printing toner.
In addition, when the flexural deformation D is generated in the photosensitive material belt 1 immediately after the nip, the effect of flexural deformation D also extends to the nip, so that defective printing may be caused as a proper nip pressure cannot be maintained.
As described, since the conventional orthogonal transfer type image forming apparatus causes flexural deformation in the photosensitive material belt as the transfer opposite roller moves, there is a problem that disturbance is caused in a printed image.